This invention relates generally to a burner assembly and more particularly to an improved burner assembly which operates in a manner to reduce the formation of nitrogen oxides and sulfur dioxides as a result of fuel combustion.
In a typical arrangement for burning coal in a vapor generator, several burners are disposed in communication with the interior of the furnace and operate to burn a mixture of air and pulverized coal. The burners used in these arrangements are generally of the type in which a fuel-air mixture is continuously injected through a nozzle so as to form a single, relatively large, flame.
In the burning of coal in this manner, unacceptable levels of sulfur dioxide are produced which must be reduced in order to meet government standards of air quality. Also, when the flame temperature at the burner exceeds 2800.degree. F., the amount of fixed nitrogen removed from the combustion supporting air rises exponentially with increases in the temperature. This condition leads to the production of high levels of nitrogen oxides in the final combustion products, which also causes severe air pollution problems.
Control of sulfur dioxide emissions is usually achieved by external means such as wet or dry flue gas desulfurization. In-situ control (i.e., within the furnace) has been under investigation for many years and utilizes either a pre-mixing of limestone (or other sorbent) with coal, or an injection of pulverized sorbent external to the burner throat through separate ports or small injection nozzles. However, both of these techniques have distinct drawbacks. The injection of the sorbent with the coal usually yields low sulfur dioxide capture ratios due to deadburning of the sorbent and can lead to increased slagging. The external injection of the sorbent requires numerous wall penetrations, tube bends and expensive piping and burner staging controls for the ports.
Also, sorbent injection between or above the burners can limit sulfur capture due to several effects:
Inadequate mixing between the products of combustion and the sorbent particles;
Insufficient residence time in the boiler's radiant zone; and
Increased slagging and sorbent deposition to the boiler's sidewalls when sorbent is injected to the lower burner levels of a multiple level boiler. This injection location also reduces sulfur capture since sorbent particles can be re-entrained in the high temperature portion of the flame.
These deficiencies can be corrected by injecting sorbent in conjunction with an internally staged low N0.sub.x burner. This type of burner reduces N0.sub.x by at least 50%, as compared to turbulent burners, without simultaneous use of external combustion air staging systems such as overfire or tertiary air ports. However, when overfire air ports are used, N0.sub.x reductions as great as 75% can be obtained. An internally-staged low N0.sub.x burner can be defined as one which yields fuel-rich and fuel-lean zones within a flame envelope similar to that of a turbulent burner. This is in contrast to delayed mixing burners which produce very long narrow flames which gradually combust the fuel over a substantially greater distance than is characteristic of either turbulent or internally staged burners.
Other attempts, including two-stage combustion, flue gas recirculation and the introduction of an oxygen-deficient fuel-air mixture suppress the flame temperature and reduce the quantity of available oxygen during the combustion process and thus reduce the formation of nitrogen oxides. However, although these attempts singularly may produce some beneficial results they have not resulted in a reduction of nitrogen oxides to minimum levels. Also, these attempts have often resulted in added expense in terms of increased construction costs and have led to other related problems such as the production of soot and the like, nor do they lend themselves to sulfur control via sorbent injection.